1. Field of the Invention
The present invention relates to a method of forming a semiconductor thin film including a process of cleaning a surface of semiconductor substrate or an insulating substrate made of glass, plastic and the like or a surface of thin film formed on such a substrate. In particular, the invention relates to a method of forming a semiconductor thin film wherein polycrystallization of amorphous films is performed through energy beam radiation as in a step of manufacturing thin film transistors (TFT) used in liquid crystal displays (LCD) and so on. The invention further relates to a plastic substrate applied to the method.
2. Description of the Related Art
A TFT is used as a device having a switching function in a TFT liquid crystal display. The TFT is formed on a substrate, corresponding to each pixel of the liquid crystal display. TFTs made of amorphous silicon (Si) and those made of polycrystalline silicon are available. A high quality poly-silicon TFT is fabricated on a substrate at a low temperature through crystallization of an amorphous silicon by radiating an energy beam such as an excimer laser beam, in particular. A pixel switching device and periphery circuitry of a liquid crystal display is formed on a single substrate, using such a poly-silicon TFT. A TFT having a bottom gate structure has particularly received attention because of its stable properties among poly-silicon TFTs.
A bottom gate TFT has a configuration as follows, for example. A gate electrode of molybdenum tantalum (MoTa) is formed on a glass substrate. An oxide film (Ta2O5) is formed over the gate electrode. A gate insulator made of silicon nitride (Si3N4) film and silicon dioxide film (SiO2) is formed on the glass substrate with the oxide film. A thin poly-silicon film is further formed on the silicon dioxide film. N-type impurities, for example, are introduced to the poly-silicon film so as to form source and drain regions. On the poly-crystalline film, a silicon dioxide film is selectively formed, corresponding to a channel region in the poly-silicon film. An n+ doped poly-silicon film is formed on the poly-silicon film and the silicon dioxide film. A source electrode and a drain electrode are further formed on the n+ doped poly-silicon film. The source and drain electrodes are electrically connected to the source region and the drain region, respectively.
The poly-silicon film of the TFT having the configuration described above may be fabricated through forming an amorphous silicon film, radiating the film with an energy beam so as to melt and cooling the film down to a room temperature.
In the steps such as polycrystallization of the amorphous silicon film through energy beam radiation in a related-art manufacturing method, however, the inventors of the invention have found that volatile contaminants deposited on the base layer of the amorphous silicon film may be vaporized through energy beam radiation. The amorphous silicon film may be thereby partially damaged.
Such volatile contaminants include water absorbed by a substrate surface or a surface of thin film formed on a substrate exposed to the atmosphere in a device fabrication process and organic substances such as residual fragments of resist when etching is performed with a resist mask and dirt deposited during handling of a substrate. If such volatile contaminants are vaporized and a gas thereby released builds up between the thin film (amorphous silicon film) and the base, the thin film may be lifted off.
In order to prevent such damage to the amorphous silicon film, the energy value of an energy beam for radiation could be reduced to a value lower than the optimum value for polycrystallization. However, the crystal thereby obtained is imperfect and not fully polycrystallized.
Besides glass, a substrate used for such a TFT may be a plastic. However, since a plastic substrate has low heat resistance, heat generated through energy beam radiation affects the substrate and causes a deformation of the substrate and so on. It is therefore impossible to radiate the optimum energy beam and to obtain a perfect crystal.
It is a first object of the invention to provide a method of forming a semiconductor thin film which is free from damage to the film and a substrate deformation with radiation of optimum energy beam for perfect crystallization.
It is a second object of the invention to provide a plastic substrate optimal for fabrication of such a semiconductor thin film.
A method of forming a semiconductor thin film of the invention includes the steps of: removing contaminants deposited on a surface of a base layer on which the semiconductor thin film is formed, through pulse laser beam radiation; forming the semiconductor thin film on the surface of the base layer free of the contaminants; and crystallizing the semiconductor thin film.
Another method of forming a semiconductor thin film of the invention includes the steps of: removing contaminants deposited on a substrate through pulse laser beam radiation; forming a gate electrode pattern on the substrate free of the contaminants; removing contaminants deposited on a surface of the gate electrode pattern through pulse laser beam radiation; forming an insulating layer on the gate electrode pattern; removing contaminants deposited on a surface of the insulating layer through pulse laser beam radiation; forming the semiconductor thin film on the insulating layer free of the contaminants; forming a source region and a drain region by selectively introducing impurities to the semiconductor thin film; and crystallizing the semiconductor thin film through energy beam radiation.
Still another method of forming a semiconductor thin film of the invention includes the steps of: forming a gas barrier layer for preventing penetration of a gas on a surface of a plastic substrate; forming a refractory buffer layer for preventing heat conduction on the gas barrier layer; forming the semiconductor thin film on the refractory buffer layer; and performing heat treatment of the semiconductor thin film through energy beam radiation.
A plastic substrate of the invention comprises a protective layer on a surface thereof. The protective layer includes a gas barrier layer having a function of preventing penetration of a gas and a refractory buffer layer of 1 xcexcm or above in thickness having a function of preventing heat conduction.
In the method of forming a semiconductor thin film of the invention, volatile contaminants are removed through radiating the base layer with a pulse laser beam whose wavelength is 100 to 350 nm, for example, as a pretreatment. Damage to the film due to vaporized contaminants deposited on the base layer surface is thereby prevented. As a result, the energy beam of the value optimal for polycrystallization is applicable in the step of crystallizing the semiconductor thin film In the other method of the invention, the refractory buffer layer between the plastic substrate and the semiconductor film prevents heat produced through energy beam radiation from affecting the plastic substrate. The gas barrier layer prevents penetration of a gas from the plastic substrate to the semiconductor film.
The plastic substrate of the invention comprises the protective layer including the gas barrier layer and the refractory buffer layer. As a result, it is possible that the semiconductor film is radiated with an energy beam of the optimal energy value.
Other and further objects, features and advantages of the invention will appear more fully from the following description.